Reporting Segments (Results of Reporting Segments) (Details) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jun. 30, 2021 USD ($) segment	Jun. 30, 2020 USD ($)
Segment Reporting Information [Line Items]
Number of reporting segments | segment					2
Noninterest Income
Trust, Investment and Other Servicing Fees	$ 1,075.4		$ 961.5		$ 2,139.1	$ 1,965.1
Foreign Exchange Trading Income	70.6		71.3		149.3	160.2
Other Noninterest Income	98.7		101.2		199.6	188.3
Total Noninterest Income	1,244.7		1,134.0		2,488.0	2,313.6
Net Interest Income	343.9		379.8		690.6	796.0
Revenue	1,588.6		1,513.8		3,178.6	3,109.6
Provision for Credit Losses	(27.0)		66.0		(57.0)	127.0
Noninterest Expense	1,120.8		1,036.9		2,238.3	2,102.5
Income before Income Taxes	494.8		410.9		997.3	880.1
Provision for Income Taxes	126.7		97.6		254.1	206.2
Net Income	$ 368.1	$ 375.1	$ 313.3	$ 360.6	$ 743.2	$ 673.9
Percentage of Consolidated Net Income	100.00%		100.00%		100.00%	100.00%
Average Assets	$ 154,300.1		$ 138,770.6		$ 153,780.8	$ 131,470.6
FTE adjustments		$ 8.3	7.7		14.9	15.8
CORPORATE & INSTITUTIONAL SERVICES
Noninterest Income
Trust, Investment and Other Servicing Fees	611.5		566.2		1,232.0	1,140.6
Foreign Exchange Trading Income	67.9		68.3		142.4	153.4
Other Noninterest Income	68.9		69.0		129.2	110.8
Total Noninterest Income	748.3		703.5		1,503.6	1,404.8
Net Interest Income	153.1		165.4		313.8	376.7
Revenue	901.4		868.9		1,817.4	1,781.5
Provision for Credit Losses	(16.8)		28.8		(22.2)	54.5
Noninterest Expense	703.6		655.6		1,416.7	1,314.9
Income before Income Taxes	214.6		184.5		422.9	412.1
Provision for Income Taxes	53.0		41.3		103.1	91.2
Net Income	$ 161.6		$ 143.2		$ 319.8	$ 320.9
Percentage of Consolidated Net Income	44.00%		45.00%		43.00%	48.00%
Average Assets	$ 119,502.3		$ 107,051.3		$ 119,820.7	$ 99,910.1
WEALTH MANAGEMENT
Noninterest Income
Trust, Investment and Other Servicing Fees	463.9		395.3		907.1	824.5
Foreign Exchange Trading Income	2.7		3.0		6.9	6.8
Other Noninterest Income	38.3		40.8		82.5	83.2
Total Noninterest Income	504.9		439.1		996.5	914.5
Net Interest Income	190.8		214.4		376.8	419.3
Revenue	695.7		653.5		1,373.3	1,333.8
Provision for Credit Losses	(10.2)		37.2		(34.8)	72.5
Noninterest Expense	398.5		372.0		801.6	766.4
Income before Income Taxes	307.4		244.3		606.5	494.9
Provision for Income Taxes	80.5		60.8		159.0	121.7
Net Income	$ 226.9		$ 183.5		$ 447.5	$ 373.2
Percentage of Consolidated Net Income	62.00%		59.00%		60.00%	55.00%
Average Assets	$ 34,797.8		$ 31,719.3		$ 33,960.1	$ 31,560.5
OTHER
Noninterest Income
Trust, Investment and Other Servicing Fees	0.0		0.0		0.0	0.0
Foreign Exchange Trading Income	0.0		0.0		0.0	0.0
Other Noninterest Income	(8.5)		(8.6)		(12.1)	(5.7)
Total Noninterest Income	(8.5)		(8.6)		(12.1)	(5.7)
Net Interest Income	0.0		0.0		0.0	0.0
Revenue	(8.5)		(8.6)		(12.1)	(5.7)
Provision for Credit Losses	0.0		0.0		0.0	0.0
Noninterest Expense	18.7		9.3		20.0	21.2
Income before Income Taxes	(27.2)		(17.9)		(32.1)	(26.9)
Provision for Income Taxes	(6.8)		(4.5)		(8.0)	(6.7)
Net Income	$ (20.4)		$ (13.4)		$ (24.1)	$ (20.2)
Percentage of Consolidated Net Income	(6.00%)		(4.00%)		(3.00%)	(3.00%)
Average Assets	$ 0.0		$ 0.0		$ 0.0	$ 0.0